The present invention generally relates to color correcting circuits for thermal printers, and more particularly to a color correcting circuit for thermal printer capable of carrying out a true color reproduction of an input image by correcting a mixing ratio of primary color inks when printing a color image.
Recently, thermal printers have been developed for printing color images based on graphic image outputs of computers, personal computers and the like. The thermal printer uses a polyester ink film having a film thickness of 5 .mu.m to 6 .mu.m, for example, and this ink film is coated on a front side thereof with ink of three primary colors, namely, yellow, magenta and cyan. The yellow, magenta and cyan inks are successively coated on the front side of the ink film for predetermined lengths each, and such successive coating is repeated. The ink coated on the ink film may be of a type which melts due to heat or a type which sublimates due to heat. The front side of the ink film is positioned to make contact with a recording paper, and a thermal printing head makes contact with a back side of the ink film. A current is applied to the thermal printing head, and the ink at a portion of the ink film corresponding to the location of the thermal printing head is transferred onto the recording paper by being melted or sublimated. The thermal printing head generally comprises a plurality of heating resistors arranged in a line, and the current is successively applied to the heating resistors depending on data of the image which is to be printed.
Accordingly, in the case where the recording paper is fixed on a platen roller, the printing is carried out with the color yellow in one revolution of the platen roller, for example, and the printing is similarly carried out with the colors magenta and cyan during respective two revolutions of the platen roller. Hence, the printing of the color image on the recording paper may be completed in three revolutions of the platen roller.
In order to compensate for a difference in the characteristic of a color separation system of an image input apparatus such as a television camera and the characteristic of a color mixing system of a color television monitor or the like so as to carry out a true color reproduction of the input image in the output image which is printed, a color correction (or masking) is generally carried out by a first degree operation process described by equation (1). EQU Y.sub.1 =k.sub.11 X.sub.1 +k.sub.12 X.sub.2 +k.sub.13 X.sub.3 EQU Y.sub.2 =k.sub.21 X.sub.1 +k.sub.2 X.sub.2+k.sub.23 X.sub.3 ( 1) EQU Y.sub.3 =k.sub.31 X.sub.1 +d.sub.32 X.sub.2 +k.sub.33 X.sub.3
In equation (1), (X.sub.1, X.sub.2, X.sub.3) denote color separated input signals of the three primary colors, such as the three primary color signals of red, green and blue of the additive primaries, and (Y.sub.1, Y.sub.2, Y.sub.3) denote color separated output signals of the three primary colors after the color correction. In addition, k.sub.11 through k.sub.13, k.sub.21 through k.sub.23 and k.sub.31 through k.sub.33 denote predetermined correction coefficients. By use of equation (1), it is possible to vary the degree of color mixture and obtain a satisfactory color reproduction.
Next, a description will be given on the case where the image output apparatus is a thermal printer using color inks which employ as the coloring material dye, pigment or the like of three primary colors of yellow, magenta and cyan which are complementary colors of the three primary colors of red, green and blue. In this case, the compensation of the characteristic of the color mixing system using the color inks is extremely complicated compared to that of the color television monitor or the like for the reasons which will be described later.
In the present specification, the three additive primary colors of red, green and blue will hereinafter simply be referred to as RGB. Similarly, the three subtractive primary colors of yellow, magenta and cyan will hereinafter simply be referred to as YMC.
FIG. 1 shows examples of the spectral reflectance characteristics of the color inks of YMC. When printing a color image, the main cause which prevents the true color reproduction of the original color document (that is, the input image) is the coloring material. In other words, the true color reproduction of the input image is mainly prevented because the spectral reflectance characteristics of the color inks are greatly deviated from the ideal spectral reflectance characteristics. For example, the yellow ink has a spectral reflectance characteristic approximating the ideal characteristic, but the magenta and cyan inks have spectral reflectance characteristics greatly deviated from the respective ideal characteristics. For this reason, it is necessary to carry out the color correction in the thermal printer so as to compensate for the deviation from the ideal characteristics.
On the other hand, FIG. 2 shows an example of a CIE-xy chromaticity diagram. In FIG. 2, a range indicated by a one-dot chain line I shows the color reproduction range of the color television monitor, a range indicated by a phantom line II shows the color reproduction range of the thermal printer which uses the color inks. As may be seen from FIG. 2, the color reproduction range of the color television monitor is large compared to that of the thermal printer, and it is possible to obtain in the color television monitor an output image having a richer perceived color. For example, the three primary colors of RGB in the color television system are described by the three primary colors of YMC in the thermal printer by overlapping the color inks, where R=Y+M, G=Y+C and B=M+C.
But the spectral reflectance characteristics of YMC shown in FIG. 1 are not ideal characteristics such that only predetermined wavelengths are absorbed and the other wavelengths reflected, and the spectral reflectance characteristics include undesirable color mixing. In addition, because the color reproduction using the color inks includes non-linear elements, it is possible to obtain only a poor color reproduction when the colors are simply overlapped, and the effect of the color correction using the first degree operation process described before is insufficient.
In order to improve this insufficient color correction, a color correcting circuit of the conventional thermal printer carries out a color correction using a second degree operation process such as that proposed in The Journal of the Institute of Television Engineers of Japan, Vol. 37, No.7 (1983), pp.546-552 (hereinafter simply referred to as Journal). In this Journal, the color correction uses a non-linear masking described particularly on p.550. According to this proposed color correction, a difference between the input image and the output image is reduced by using the method of least squares. The color reproduction in the output image within the reproducible range of the color inks is improved by expanding equation (1) to the second degree terms as shown in equation (2), where (a.sub.ij) denotes a coefficient matrix of the correction coefficients, i=1 to 3 and j=1 to 10. ##EQU1##
However, in the color correcting circuit of the conventional thermal printer, when the input RGB signals of the color television monitor are used as the color separated input signals, for example, the color reproduction cannot be carried out with the color inks in a range III shown in FIG. 2 which is within the color reproduction range I of the color television system but is outside the color reproduction range II of the color inks. When equation (2) is applied as it is to the range III in which the color reproduction is not possible by use of the color inks, an extreme change may occur in the hue, and such application of equation (2) is impractical in this case.
In other words, as stated in the Journal, an approximating equation is obtained by the method of least squares between the color separated input signals of the thermal printer and the printed output so as to obtain a satisfactory color reproduction, but the approximating equation cannot be applied to the color input signals in the range outside the color reproduction range of the color inks.
It will be assumed for convenience' sake that X.sub.2 .congruent.0 and X.sub.3 .congruent.0 in equations (1) and (2). In this case, the relationship between the color separated input signal X.sub.1 and the color separated output signal Y.sub.1 becomes a linear characteristic k shown in FIG. 3 according to the first degree operation process of equation (1), while the relationship becomes a non-linear characteristic l shown in FIG. 3 according to the second degree operation process of equation (2).
The characteristic l is obtained by looking only within the color reproduction range II of the color inks, and a hatched portion A shows an example of a portion where a change in the color output is non-uniform with respect to the color input existing outside the color reproduction range II.
The colors which are printed and reproduced are described by overlapping the color separated input signals X.sub.1, X.sub.2 and X.sub.3. Generally, when a color of high saturation exceeds the color reproduction range of the color inks, either the level of one color in the color separated input signals X.sub.1 through X.sub.3 is extremely high compared to the other color components, or the level of one color in the color separated input signals X.sub.1 through X.sub.3 is extremely low compared to the other color components. For example, in the former case, the operation result of the second degree correction (hereinafter simply referred to as a second degree correction operation result) becomes abnormal as indicated by the hatched portion A on the characteristic l in FIG. 3, and a correct color reproduction cannot be achieved when a color correction is carried out based on the second degree correction operation result.
For example, it is possible to conceive a method of obtaining an approximating equation for carrying out the color correction uniformly within the color reproduction range I of the color television system shown in FIG. 2. However, when the color correction is carried out based on such an approximating equation, it is impossible to obtain a satisfactory approximation because the reproduction error occurs differently within and outside the color reproduction range II of the color inks.
On the other hand, it is also possible to consider a method of calculating before the printing the positions of the colors corresponding to the color separated input signals on the x-y coordinate shown in FIG. 2, and changing the approximating equation based on the calculated positions on the x-y coordinate. But in order to carry out the calculation at a high speed by use of a random access memory (RAM), for example, it is necessary to have a parallel input of 18 (=6.times.3) bits when the gradation level of each color is described in 6 bits, for example. As a result, the RAM must be a large scale memory of 6 (.congruent.2.sup.18 .times.8.times.3) Mbits, and there is a problem in that this method is impractical from this point of view.